Recently, the use of VMs (Virtual Machines) have spread widely along with the advancement of virtualization technology, a typical example of which is a VDI (Virtual Desktop Infrastructure). In order to utilize VMs speedily, it is common to copy virtual disk files having installed OS (Operating System) environments.
However, since the size of virtual disk files may be as large as a few GB (Giga Bytes), the process for physically copying the files may take approximately tens of seconds to even tens of minutes. Therefore, a file cloning technique capable of virtually copying the virtual disk files so as to cut down the time required for the copying process is drawing attention (patent literature 1).